Light emitting diodes (LEDs) are applied to provide a wide variety of illumination solutions, e.g., environmental lighting and backlight in mobile devices, because they offer advantages on energy consumption, lifetime, physical robustness, size and switching rate. The LEDs are electrically coupled to a certain configuration (e.g., a string or an array) to enable a desirable illumination level. Oftentimes, the LEDs are driven by an enhanced LED voltage that is raised from a regular power supply voltage by a boost type voltage regulator. The enhanced LED voltage is specifically determined according to the configuration and operation voltages of the LEDs. This boost type voltage regulator is implemented as a switch mode power supply (SMPS). When its duty cycle varies, the SMPS allows the enhanced LED voltage to be modulated within a voltage range thereof, thereby enabling the dimming effect on illumination delivered by the LEDs.
However, the boost type voltage regulator merely allows a limited voltage variation for the enhanced LED voltage, and cannot be used to drive the LEDs and provide a desirable illumination level when the coupling configuration of the LEDs varies (e.g., the number of the LEDs coupled within a string or array increases or decreases). For example, when a subset of LEDs are decoupled from a LED string, the enhanced LED voltage has to be reduced by one or more diode junction voltages, which could not be provided by the small voltage variation of the enhanced LED voltage. In this situation, the voltage regulator would fail to drive the LEDs having the new coupling configuration, because the enhanced LED voltage would overdrive and damage the LEDs. It would be beneficial to have a more flexible and tolerant LED driver than the current practice.